1. Field of the Invention
The present invention relates to a projection lens suitable for the projection examination of an object. More particularly, the present invention relates to a projection objective lens of wide field angle and low magnification which gives an accurately enlarged image of an object to be examined.
2. Related Background Art
For the project examination instrument, it is generally required to form an accurately enlarged image of an object on a screen. It is also essential for the projection examination instrument to exclude measurement error attributable to accidental error such as focusing error as completely as possible.
In view of the above points, the projection object lens should be formed as a telecentric optical system and the distortion of the lens should be corrected sufficiently. Of course, like other lenses such as microscope objective, the projection lens should be corrected well also regarding other aberrations.
The above requirements are generally applicable to all of projection objective lenses. However, when man tries to make a projection lens of low magnification such as above five magnifications (.times.5) which can satisfy the requirements, he encounters the following difficult problems:
The first problem concerns the working distance of the lens (the distance from the object plane to the front end of the lens). In general, the working distance of an object lens is about a half of the focal length. Therefore, if this is applied to a low magnification projection objective lens having a long focal length, the working distance will be too long and the lens size (the distance from the object plane to the rear end of the lens) will be extraordinarily large. The objective lens for use in projection examination apparatus is desired to be small in size and compact. To satisfy the requirements of sufficiently long working distance and small lens size at the same, the low magnification objective lens has usually been formed by employing the so-called tele-type (telephoto type) construction comprising a converging lens unit and a diverging lens arranged in this order from the image side.
In this case, however, it is impossible to form a telecentric optical system employing a mere basic tele-type construction in an objective lens. According to the prior art, a converging field lens unit is additionally disposed between the diverging lens unit and the object plane to form the telecentric optical system. An example of such construction of low magnification projection objective lens is disclosed in Japanese patent application laid open No. 204,816/1984.
Since the low magnification objective lens has a large field diameter, the load which the field lens unit has to bear for good correction of distortion is very large. For this reason, in the prior art low magnification projection objective lens as disclosed in the above-referred Patent Publication, Japanese patent applicaton laid open No. 204,816/1984, many lenses have been used in the field lens unit. Even by it there remains some aberration to be corrected. Acccording to the prior art, the tele-type converging and diverging lens units have been used to correct the remaining aberration. The refractive powers of these lens units have been rendered strong by it and the number of lenses needed has been increased. Because of it, the low magnification projection objective lens has a very complicated construction. For such complicated construction, the lens performance of it is still unsatisfactory.
Second problem relates to reflection illumination.
Projection lenses used in the art at present generally contain a half-mirror for guiding illumination light to the object to be examined. This built-in half mirror improves the easy operability of the projection system. Even in the case of the above-mentioned prior art tele-type lens construction, it is possible to incorporate a half-mirror into the projection lens when the refractive power is suitably distributed among the lens units. However, when a half-mirror is incorporated into the projection lens, flare is caused by it. In the case of reflection illumination, the object surface is illuminated through the above-mentioned field lens units of the projection lens. At the time, a flare is formed by the light reflected by the lens surface of the field lens unit. Since, as previously mentioned, the projection lens of low magnification, for example, that of five magnifications has a large field diameter, the lens has to have a large lens diameter in order to maintain the telecentricity at least on the object side.
As aberrations, in particular distortion must be corrected very well, it is unallowable to give a large curvature to the lens surface. Under the conditions, flare is easily formed.
The problem of flare is enhanced by the fact that the refractive power of the field lens unit necessary as the third lens unit must be fulfiled by many lenses each having a weak refractive power. The observation of the object is disturbed by the flare very much in particular when the object to be observed is an object of low reflectance. If the number of the lenses used in the field lens unit is decreased in order to reduce the flare, the share of aberration correction falling to the lens unit is rendered too large. It is no longer possible to correct distortion and other aberrations adequately.
Third problem concerns the manufacturing cost of the projection objective lens.
For the telecentric optical system it is required to use, as the object side lens, a lens whose diameter is larger than the object diameter at least. For example, in the case of an objective lens of five magnifications (.times.5) for a projection examination instrument having a screen diameter of 350 mm, the field diameter is 70 mm and, therefore, the lens needed is such a large lens whose diameter is 80 mm at least taking the aperture into account.
Since such a large lens is required, the total manufacturing cost of the projection objective lens is remarkedly increased up with increasing the number of lenses used in the above-mentioned field lens unit.